The present disclosure relates generally to industrial power plant machinery and, more particularly, to a method and system for determining a lean blow out condition for gas turbine combustion cans.
Gas turbines generally include a compressor and turbine arranged on a rotating shaft(s), and a combustion section between the compressor and turbine. The combustion section burns a mixture of compressed air and liquid and/or gaseous fuel to generate a high-energy combustion gas stream that drives the rotating turbine. The turbine rotationally drives the compressor and provides output power. Industrial gas turbines are often used to provide output power to drive an electrical generator or motor. Other types of gas turbines may be used as aircraft engines, on-site and supplemental power generators, and for other applications. Certain gas turbines include several tangentially located combustor cans that burn fuel in high-pressure compressed air to isobarically raise the temperature of the resulting gaseous mixture. The resulting hot gas is fed to a multi-stage turbine (known to those skilled in the art as a combination of nozzles and buckets, or stators and rotors in each stage), where the gas performs the work for generating electricity, for example.
Fuel and air flow rates are controlled in order to allow for ignition of a flame in the burner as well as for clean emissions after ignition. A burner controller is used to control the fuel and air flow rates provided by a fuel regulator and blower, respectively. The fuel regulator is typically set to an initial value for ignition. Once the flame is proved, the burner controller varies the fuel flow rate to control the heater head temperature, as measured by a head temperature sensor. A flame is proved when a flame detector detects the presence of the flame. There are several types of flame detectors including thermocouples and ultraviolet sensors known in the art.
The output (or air mass flow rate) of the combustion air blower is set by the burner controller to control the fuel-air ratio in the combustion chamber. The fuel-air ratio is the ratio of the fuel mass flow rate to the air mass flow rate and is a primary factor affecting emissions. The blower controls the fuel-air ratio by increasing or decreasing the air mass flow rate relative to the fuel mass flow rate. For example, in order to hold the fuel-air ratio constant, the burner controller will increase the blower output as the fuel regulator increases its output and vice versa. The desired fuel-air ratio and the fuel flow rate may be changing at the same time, so the burner controller will change the output of the blower to accommodate both the change in desired fuel-air ratio and the fuel flow rate.
Minimizing the emissions of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) requires a lean fuel-air mixture that still achieves complete combustion. A lean fuel-air mixture has more air than a stoichiometric mixture (i.e., 15.67 grams of air per gram of fuel, if propane is used, for example). However, as more air is added to a fixed amount of fuel, the emissions of CO, HC and NOx will decrease until the fraction of air in the fuel-air mixture is large enough that the flame becomes unstable. At this point, pockets of the fuel-air mixture will pass through the burner without complete combustion. Incomplete combustion of the fuel-air mixture produces large amounts of CO and HC, which will quickly increase as more air is added to the fuel-air mixture until the flame extinguishes at a lean blow-out limit (“LBO”). The LBO will increase as the temperature of the incoming air (i.e., the preheated air) increases.
The fuel-air ratio must first be controlled to provide the optimal fuel-air ratio for ignition. Once the flame is proved, the fuel-air ratio is controlled to minimize emissions based upon the temperature of the preheated air and the fuel type. When the fuel flow rate is increased or decreased to adjust the temperature of the heater head, the air flow rate is also adjusted to maintain the desired fuel-air ratio.
A given fuel will only ignite over a limited range of fuel-air ratios. At ignition, an ignition fuel-air ratio is chosen which is slightly above or below the stoichiometric fuel-air ratio corresponding to the fuel being used. As mentioned above, use of a lean fuel-air mixture minimizes the emissions of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx). Typically, however, lighting a lean pre-mixed fuel-air mixture can be difficult.
Accordingly, it would be desirable to have a real time method for determining and predicting when a combustor is running close to its lean blow out limit, and thereafter generate an automated warning signal such that either manual or automated corrective action can be taken to correct the problem and prevent blowing out of the combustion system.